Modeling tools aid in immersion lithography quest

Portland, Ore. - A German company is fielding modeling tools that can help chip makers assess the promise of immersion lithography. Putting a layer of water between a wafer and the stepper lens could extend current 193-nanometer lithographic exposure tools down to the 45-nm chip-manufacturing node and below, experts believe.

"People want to decide this year whether to scrap their plans to go to some next-generation lithography technique like EUV [extreme ultraviolet], or to use immersion lithography," said Peter Brooks, applications support engineer with Sigma-C GmbH (Munich, Germany). "With EUV, you need to expose wafers in very clean rooms, but with immersion you can just put a puddle of water between the lens and the wafer."

To help chip manufacturers make that decision, Sigma-C recently upgraded its Solid-C simulator to include "one-click" immersion. "The nice thing about our simulator is that you can click the mouse to turn immersion on and off," said Brooks. "You can try out 193 nm with immersion and compare it to 157 nm dry before you commit to anything."

The company also models increased depth of focus. Sigma-C simulates three-dimensional wafer stack topography in conjunction with liquid immersion lithography and determines how the stepper will image the mask into the wafer, outputting a three-dimensional resist "profile." From that engineers can measure to determine if their process is correct-all before a single chip is fabricated.

"Sigma-C answers the question 'for a given feature on the mask, this is how it will print,' " said Brooks. "We can simulate hot spots where engineers suspect that there are going to be problems and tell them whether they will be a problem."

For now, no exposure tools from big vendors offer lithography that places fluid between the lens and the wafer, but ASML Holding NV, Canon Inc. and Nikon Corp. are all developing 193-nm tools with immersion capabilities. Many chip makers are looking at the technology, with only Intel Corp. dissenting that immersion is too little, too late.

Taiwan Semiconductor Manufacturing Co. disclosed in April that it was pursuing immersion lithography for chip production at the 65-nm node. Reportedly, TSMC is working on the technology with its sole lithography-tool supplier, the Dutch giant ASML. In July, the chip-making consortium International Sematech (Austin, Texas) gave immersion lithography the green light for development (see www.eetimes.com/at/se/news/OEG20030711S0016).

The total available market for immersion lithography tools is projected by The Information Network to go from nothing to $230 million by 2005.

For years, microscopy has used oil between specimen and lens to enhance resolution. Now chip makers are considering a similar technique. In immersion lithography, the space between the lens and the wafer fills with a liquid; water works for 193-nm exposure tools, but 157-nm tools will need oil.

Air is the worst medium, because its 1.0 index of refraction means lots of bending when light leaves the glass. But since the index of refraction is 1.48 for water and 1.52 for glass, much less bending is required, giving both a sharper (theoretically down to 35 nm) and deeper focus. In effect, immersion technology offers better resolution enhancement and higher numerical apertures over conventional projection lithography.

Sigma-C's simulator models the greater light-gathering capability of immersion, as well as the finer, deeper lines it produces. "By adding the liquid, you enable the lens to be bigger by actually changing its numerical aperture, because it is gathering previously lost rays from the edge of the lens to the center," said Brooks.

It also models the shortcomings of immersion lithography. One obstacle still to be overcome by stepper manufacturers is transverse magnetic polarization, whereby improperly polarized waves from opposite sides of the lens can meet in the resist at 90 degrees and zero out the contrast-especially for alternating phase-shift masks, which depend on light from the edge of the lens to make proper exposures.

"The good thing is that if you put in the wrong polarization values, then with our simulator you can see those zero-contrast effects right away," said Brooks.